Every year there are about 300-500 million clinical cases of malaria. About 40% of the world's population is at risk of acquiring the disease. (Croft (2000) BMJ 321:154-160.) Malaria is characterized by headache, malaise, anemia, splenomegaly, and paroxysms with cold, hot, and wet the stages. (Winstanley (1998) Journal of the Royal College of Physicians of London 32(3):203-207.) Hemolysis and ischemia cause the majority of the symptoms seen with acute malaria. Malaria is caused by protozoa of the genus Plasmodium. There are over 100 species of which 22 infect nonhuman primates and 82 are pathogenic for reptiles and birds. The four species that commonly infect man are: P. falciparum, P. malariae, P. vivax, and P. ovale. Malaria may be transmitted by a bite of the Anopheles mosquito, infected blood transfusions, transplacentally, and in laboratory inoculation accidents.
Plasmodia have a complex life cycle where the sexual phase occurs in the Anopheles mosquito and the asexual phase takes place in the veterbrate host (i.e. a human). (Randall, et al. (1985) Pediatric Clinics of North America 32(4):893-916.) The process of sexual reproduction in the mosquito is called Sporogony and includes the period from gametocyte maturation to sporozoite development. When a female Anopheles mosquito feeds, it takes up gametocytes present in the blood of an infected host. The gametocytes taken up by the mosquito pass to the mosquito's gut. A zygote is formed by the fusion of the microgamete and macrogamete. After 12 to 24 hours, the zygotes elongates and becomes motile and is called an ookinete. The ookinete later penetrates the mosquito's stomach to form an oocyst which divides into thousands of spindle-shaped sporozoites which are released throughout the mosquito's body.
When a blood meal is taken by an infected Anopheles mosquito, sporozoites from the salivary glands of the mosquito are inoculated into the bloodstream of the veterbrate host (i.e. human) and are carried to the liver. At the end of the hepatic phase of development, thousands of merozoites are released into the circulation where they bind to and enter red blood cells. The erythrocyte phase of asexual reproduction is termed Schizogeny. When the infected erythrocytes rupture, they release merozoites which can invade more red blood cells. Other released merozoites become gametophytes capable of infecting feeding mosquitoes and restarting the life cycle of the Plasmodia.